The present invention relates to a surface acoustic wave device (SAW device), and in particular to a surface acoustic device using a withdrawal weighted electrode.
As electrodes for surface acoustic wave devices such as SAW filters, electrodes of normal type, in which comb-like electrodes each having electrode width equivalent to gap width are interdigitated with equal finger overlap length everywhere, have conventionally been used.
If such an electrode of normal type having a uniform electrode finger overlap is used, it is known that a surface acoustic device has a frequency response H(f) represented by the following equation as described in Impulse Response Model Design of Acoustic Surface-Wave Filters (C. S. Hartmann et al.; IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, vol. MTT-21, No. 4, pp. 162-175, April, 1973) EQU H(f)=(sinX)/X*exp(-j.omega.N/2f.sub.0) EQU X=N.pi.(f-f.sub.0)/f.sub.0
where N, .pi., f, f.sub.0 and .omega. denote the number of pairs of the electrode, the ratio of the circumference of a circle to its diameter, frequency, center frequency and angular frequency, respectively.
In case a surface acoustic wave device using such an electrode of normal type is utilized as a filter, however, small attenuation of side lobes brought about inconvenience in practical use.
On the other hand, overlap length weighted transducers having adjacent electrode fingers of comb-like electrodes changed in overlap length (as described in U.S. Pat. No. 3,663,899) are known as a technique for improving the attenuation of side lobes.
However, such overlap length weighted electrodes tend to be affected by diffracted waves. In general, overlap length weighted electrodes can be used in only one of two interdigital transducers provided in a surface acoustic wave device, and the other one of the two interdigital electrodes must be the above described electrode of normal type. That is to say, if a multistrip coupler is used, the above described overlap length weighted electrodes can be used in both of two interdigital electrodes of a surface acoustic wave device. In that case, however, the chip dimension of a piezoelectric surface acoustic wave substrate becomes large (F. G. Marshall et al., IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, vol. MTT-21, No. 4, pp. 206-215, 1973).
Therefore, a technique of a surface acoustic wave device in which parts of impulse excitation sources are removed by partly cutting away fingers of comb-like electrodes, i.e., a technique of a surface acoustic wave device using withdrawal weighted electrodes has been proposed (C. S. Hartmann; 1973 Ultrasonic Symposium Proceedings, pp. 423-426, 1973).
Such a surface acoustic wave device using withdrawal weighted electrodes can be used jointly with overlap length weighted electrodes without needing a multistrip coupler.